Project Summary Infants are faced with the task of learning language in noisy environments. It is well known that infants have greater difficulties listening in noisy situations than adults, even though their ability to encode sounds appears to be mature. One possible explanation is that infants have greater difficulties segregating simultaneous sounds. The sound components of different sound sources arrive at the ears as a complex sound mixture. The task of the auditory system is to group sound components together that belong to the same sound source and separate those that do not. Adults accomplish this task by using a variety of auditory cues, such as frequency proximity or onset asynchrony. Central processes and selective auditory attention also play an important role. Evidence on how infants solve this task is sparse but indicates that infants are able to use some of the same auditory cues that adults use. However, temporal cues, which are strong cues to sound source segregation in adults, have not been investigated in infants. Given that central auditory processes develop rapidly within the first year of life developmental differences would be expected. The overall goal of this research is to test hypotheses about infants' ability to use auditory cues to separate concurrent sounds. The observer-based psychoacoustic procedure and a modified double-vowel paradigm will be used. Three- and 7- month-old typically developing infants and adults will be included. The first specific aim of this research is to investigate whether infants are able to use two different temporal cues to segregate concurrent speech sounds. The cues that will be investigated are onset asynchrony and amplitude modulation. It is expected that infants and adults will be able to separate concurrent speech sounds better with a temporal cue and that this ability will increase with age. In infants, amplitude modulation is expected to improve vowel segregation more than onset asynchrony, a pattern not expected for adults. In addition to investigating isolated cues, this research will also test how infants use cue combinations. Natural environments typically include a multitude of auditory cues and adults change the cues they use based on what cues are available. The second specific aim of this research is to investigate how infants and adults weigh onset asynchrony and amplitude modulation when both cues are available. It is expected that infants and adults will be better able to segregate the simultaneous speech sounds when two cues are available, and that the benefit of combining cues is relatively greater for infants than for adults. The results of this research will contribute to the growing understanding of the development of auditory skills in complex listening environments. Furthermore, the results will lay the groundwork to investigate how auditory development in infants with hearing loss differs from infants with typical hearing. This can lead to the development of better treatment and intervention approaches.